When a high gain amplifier is used in a closed loop system, such as a phase-lock loop, and the system is in a non-linear mode, such as occurs when a phase-lock loop is out of lock, the high gain amplifier typically reverts to a low gain mode. This occurs when loop transients and beat note signals change the amplifier's output to a voltage close to one of its supply voltages, either positive or negative, from the amplifier's normal output voltages. A term used by those skilled in the art to define this condition is "going to the rails".
One example of the above is a system where a Type 2 second order phase-lock loop is attempting to recreate a clock signal from an incoming data signal. The phase-lock loop typically incorporates a phase detector of some sort, which provides an output signal through an active loop filter containing a high gain amplifier to drive a voltage controlled oscillator to a phase and frequency which is identical with that of the clock originally used to clock the data of the incoming signal being compared. In normal operation, the gain of the amplifier is quite high and is at an output value which is some median point between power supply voltages. When the phase-lock condition is interrupted due to some influence such as noise or loss of signal to be compared, a sweep signal outside normal loop operation may be utilized to reestablish a phase-locked condition. However, if the sweep signal is summed with the output of the high gain amplifier to be applied to the VCO, and if further the high gain amplifier has gone "to the rails", the rail voltage interferes with the prompt reacquisition of synchronization.
A known prior art attempt to correct this condition is to connect opposite polarity diodes in parallel and to connect that combination from the output to the input of the high gain amplifier as a type of electronic clamp. Such a solution may work to limit the output excursion of the amplifier, but such a solution cannot hold the output of the high gain amplifier closer than an integral number of diode drops away from the final value so a slower sweep rate, higher amplitude sweep voltage is required and, thus, reacquisition of phase lock is made slower. Where the high gain amplifier comprises part of a filter network as well, the parallel connection of the diodes may affect the closed loop transfer function by shunting a critical network with a finite leakage path.
The present invention overcomes the limitations of the prior art by activating a circuit which is operational only when there is a detection that the phase-lock loop is no longer in a phase-locked condition, and providing a feedback signal to the input of the high gain amplifier to keep it driven towards a central value as ascertained by a pair of reference voltages on either side of a "normal" output voltage of the high gain amplifier. As soon as phase-lock is reestablished, the feedback circuit is deactivated and the system can revert to normal operation.
It is thus an object of the present invention to provide an improved approach to prevent a high gain amplifier in a closed loop system from reverting to a low gain condition and to limit the excursions of the output of said amplifier.
Other objects and advantages of the present invention will be apparent from a reading of the specification and appended claims in conjunction with the single drawing, which is a combination block-schematic diagram of one embodiment of the invention as applied to a phase-lock loop system.